Propylene is for more than 50% produced by steam cracking processes. Typical feedstock is straight run naphtha as obtained when refining a crude petroleum source which typically comprises of unsaturated compounds, like paraffinic and naphthenic compounds, and aromatic compounds. Steam cracking processes are very complex and for this reason alternative processes starting from naphtha feedstock have been described in US2010/0274063 and US2010/0331590. These processes involve a dehydrogenation step to increase the olefin compound contents followed by a olefin cracking step wherein propylene is formed.
Propylene is also prepared in a refinery environment as a by-products of the Fluid catalytic cracking (FCC) process. Since the late nineties, some FCC units have been operating at higher severity to achieve a propylene yield of 10-12 wt percent of the fresh FCC feed. To further increase the propylene yield, different processes have been developed around the FCC configuration in a refinery and it has been reported that propylene yields up to 20 wt % of fresh FCC feed have been achieved.
One way to increase the propylene yield is to add a medium pore zeolite to the FCC catalyst as for example described in DE4114874. Various variants have been developed wherein the medium pore catalyst and the FCC catalyst contact the hydrocarbon fractions in riser reactors. A development is a FCC process employing two risers as for example described in US2013158326. A disadvantage of these processes is that the medium pore zeolite catalyst will be subjected to a regeneration step together with the FCC catalyst which causes the medium pore zeolite catalyst to degenerate. A disadvantage of the dual risers is the complexity of the process and the inflexibility to control the yield of propylene in response to changing economics. This may be the reason that up to this moment very few dual riser FCC units have been build.
US2012/0071701 describes a process wherein a fraction comprising olefins and paraffins in the C4-C12 range as obtained in a FCC process is first contacted with a olefin conversion reactor. From the effluent of the olefin conversion reactor light olefins, like propylene, are isolated. The remaining fraction is sent to a dehydrogenation reactor. The effluent of the dehydrogenation reactor is recycled to the olefin conversion reactor. According to the description the olefin conversion reactor and the dehydrogenation reactor may be a fixed bed reactors, fluidized bed reactors or a continuous catalyst regeneration (CCR) system.
A disadvantage of the process of US2012/0071701 is the high coke formation in the dehydrogenation cracking reactor. This coke formation forces one to perform the dehydrogenation in a fluidised bed reactor applying a separate regeneration reactor resulting in a complex process.
WO03/082462 describes a process to prepare light olefins by contacting a feedstock with a catalyst having a dehydrogenation function, as provided by a V/Mg metal oxide component, and a cracking function, as provided by a SAPO-34 zeolite. The feed was contacted with this catalyst in a fixed bed downflow reactor. Although the use of a fixed bed is favourable the coke make of such a process is relatively high. This will result in that the catalyst will have to be regularly decoked. Furthermore when starting from mixtures of hydrocarbons also containing aromatics more coke forming is to be expected.
The aim of the present invention is to provide a simpler process for the preparation of propylene starting from a mixture of hydrocarbons comprising aromatic hydrocarbon compounds as may be obtained from a FCC process.